1. Field of the Invention
The present invention relates to a zoom lens and an information device. The zoom lens according to the present invention can be suitably used for a digital camera, and also can be suitably used for a video camera and silver salt camera. The information device according to the present invention can be performed as a digital camera, video camera, silver salt camera, etc., and also can be performed as a personal digital assistance device.
2. Discussion of the Background
Recently, there has been widely spreading a digital camera and “a personal digital assistance device having a photographing function”. A user has a wide variety of demands with respect to those devices. Especially, a user always requires high quality of a photographed image and a downsized device, and also requires both high performance (implementation of high resolution, wide field angle and larger changing magnification) and downsizing with respect to a zoom lens, which is used as a photographing lens.
In terms of the downsizing of zoom lens, it is necessary to reduce the entire length of the zoom lens (the distance from the lens surface of the most object side to the image surface) when used. It is also important to reduce the thickness of each lens group to control the entire length when collapsed.
In the high performance of zoom lens, it is necessary to have high resolution corresponding to an image pickup device having at least 4 million pixels, desirably, 8 million pixels or more over “the entire zoom area” from the aspect of resolution.
Moreover, there are many users requiring the wider field angle of zoom lens. In terms of the wider field angle, it is desirable that “a half field angle at the wide angle end: 38 degrees or more”. “The half field angle: 38 degrees” corresponds 28 mm with “the focal length of conversion of 35 mm silver salt camera (i.e., Leica)”.
Furthermore, relatively larger changing magnification ratio is desired, but in terms of larger changing magnification ratio, it is considered that general photographing can be mostly performed if it is a zoom lens corresponding to about 28-135 mm (about 4.8 times) with the focal length of conversion of 35 mm silver salt camera.
Conventionally, as a zoom lens having a four-groups structure, there has been disclosed a zoom lens comprising a first lens group having a positive refracting power, a second lens group having a negative refracting power, an aperture stop, a third lens group having a positive refracting power, and a fourth lens group having a positive refracting power, sequentially arranged from an object side, and during changing the magnification from the wide angle end to the telephoto end, the first lens group and the third lens group being monotonically moved to the object side, the second lens group being fixed, and the fourth lens group being moved (reference to JP-A-H04-190211).
Moreover, similar to the above four-groups structure, there has been disclosed a zoom lens that the first lens group and the third lens group monotonically move to the object side, the second lens group monotonically moves to the image side, and the fourth lens group moves, during changing the magnification from the wide angle end to the telephoto end (reference to JP-A-H04-296809).
Furthermore, similar to the above four-groups structure, there has been disclosed a zoom lens that the first lens group and the third lens group monotonically move to the object side, and the second lens group moves to the object side after moving to the image side, during changing the magnification from the wide angle end to the telephoto end (reference JP-A-2003-315676, JP-A-2004-212616, JP-A-2004-212618, and JP-A-2004-226645).
In the zoom lenses disclosed in JP-A-H04-190211 and JP-A-H04-296809, there is still room for improvement in the wider field angle because the half filed angle at the wide angle end is about 25-32 degrees.
In the zoom lenses disclosed in JP-A-2003-315676, JP-A-2004-212616, JP-A-2004-212618, and JP-A-2004-226645, the half field angle at the wide angle end: about 34-37 degrees is proposed; however, there is still room for improvement in the larger changing magnification because the changing magnification ratio is over 4 times from the aspect of changing magnification.